BARBADOS-ANTIGUA REPORTS 35 



Thus a group of animals represented in a given region wholly 

 or chiefly by a number of very distinct types, or monotypic gen- 

 era, has been more or less isolated in that region for a very con- 

 siderable period. 



It is interesting to compare the occurrence of the ophiurans in 

 the Caribbean region with that of the crinoids. 



Total number of ophiuran families, 13 ; of crinoid families, 28. 

 Total number of ophiuran genera, 192 ; of crinoid genera, 142. 

 Total number of ophiuran species, 1420 ; of crinoid species, 576. 

 Ophiuran families in the West Indies, 13; percent of total, 

 100. 



Crinoid families in the West Indies, 16 ; percent of total, 54. 



Ophiuran genera in the West Indies, 88 ; percent of total, 46. 



Crinoid genera in the West Indies, 30; percent of total, 21. 



Ophiuran species in the West Indies, 255 ; percent of total, 18. 



Crinoid species in the West Indies, 51 ; percent of total, 9. 



Endemic ophiuran genera, 21 ; percent of total in West In- 

 dies, 24. 



Endemic crinoid genera, 16 ; percent of total in West Indies, 

 53. 



Monotypic ophiuran genera, 18 ; percent of total in West In- 

 dies, 20. 



Monotypic crinoid genera, 11 ; percent of total in West Indies, 

 37. 



The recognized families and subfamilies in the recent crinoids 

 are in almost all cases better differentiated than the same divi- 

 sions among the ophiurans as we understand them now. This 

 indicates a phylogenetic advancement of the crinoid over the 

 ophiuran fauna wherebj' the former has become more definitely 

 crystallized into well circumscribed types. 



The relationships of the ophiuran and crinoid families, genera 

 and species may be anah'zed as follows : 



OPHIURANS CRIXOroS 



Number of families 13 28 



Average number of genera per family 16 5 



Average number of species per genus 7 4 



